I imagine that at the moment it really is in the "Just to show they can" phase. I can anticipate some potential benefits of this kind of manufacture, though I'm not an expert in the field, so take it with a pinch of salt! There was an episode of Grand Designs on UK television (I think it's a Channel 4 show) where they build a house using insulation-filled plywood boxes that were CNC-cut on site. It's probably worth a watch if you can get it, since it must have similar benefits to printing (e.g. just ship raw materials to the site and then any custom parts are made just-in-time). What I've heard about that episode has put ideas in my head about that sort of technique...
Firstly, assuming a more completely automated system than the one described above, you might build a house by simply assembling a 3D printer machine around the site and letting it run. You could potentially assemble a building very quickly and with a relatively small workforce.
Secondly, as with other forms of 3D printing, there's the potential to build an irregularly-shaped or custom-designed structure just as easily (in terms of construction effort) as a standard one. Less need to mess about with what curves you can easily make, getting the right sized construction materials, etc. Just let the machine lay down walls in whatever configuration you want.
Thirdly, even in a system like the one described (printing out elements which then require finish and - presumably - putting into place) you still get the opportunity to make custom components on-site where they'll be used. Raw material can be shipped to you in a dense form, instead of transporting unwieldy, finished parts. Although you incur the cost of using the 3D printer, you do avoid the cost of having someone in a factory construct moulds for the concrete shapes you need. I can also imagine designs being checked during the build and minor alterations made before running off the next part. Design errors might be fixed in this way without greatly delaying the project.